Distillate rectification



Jan. 5, 1943. s. c. CARNEY DISTILLATE RECTIFICATION Filed March 19, 1940U PUDQORE m PUDQONE Ll 7% .m mwnEEhm mummomua Y E R 0M N 9 MR 0 EA T Q mWC mwmmomuo mummomma s z I] Y B 2 m M 1 v w m h o. m /9 m7 w PatentedJan. 5, 1943 UNETED STATES ATENT OFFICE DISTILLATE RECTIFICATIONDelaware Application March 19, 1940, Serial No. 324,895

4 Claims.

The present invention relates to a method of and apparatus forseparating gases, vapors, and liquids. More specifically, this inventionrelates to the removal of undesirable gases from a mixture.

In the process of separation of petroleum into commercial products,there are many instances wherein the removal of undesirable componentsis either necessary to or greatly assists in the separation of themixture being processed into fractions having desirable qualities formarketing or for feed stock to other processes. For example, hydrogen,hydrogen sulfide, methane, and inert gases are common to most crude oilsand must be removed prior to or during refining. In the natural gasolineindustry, the product must be free from the lower boiling hydrocarbonsto be stable for storage. The present invention is described herein asapplied to the rectification of the condensate obtained from distillatetype wells, but it is to be understood that the invention is applicableto other problems of removal of undesirable gases.

The present recycling operation on distillate wells makes the condensatein a high pressure accumulator at a pressure of 500 to 1400 pounds persquare inch by reduction of the higher well pressure, and either with orwithout cooling by refrigeration from sources other than expansion ofthe gas. At present, practice is not yet settled on what is the bestaccumulator pressure. Condensates from the high pressure accumulatorsare handled in practice by stage venting in three or more flashingoperations. Stage venting effects the removal of methane but asubstantial amount sure the heptane and heavier present require a kettletemperature impractically high.

Even if it were practical to use the conventional rectification of thenatural gasoline industry after reasonably flashing off of methane, thegreat loss,

of polymerization feed especially, has happened during the stageventing. It is known, and practiced to some extent, to recover desiredcomponents from the flashed gas by the use of oil absorption, but evenwith absorption, the situation is complicated. There are usually threeflashed gases at different pressures and richer in the heaviercomponents as the pressure declines. The rich gases are so rich as tooverheat an absorber, whereas the high pressure vent gas will, due toits pressure, cause considerable methane to be dissolved in the oil. Inapplying conventional absorption, either the three gases may be mixed inone absorber with consequent compression of the righ gases and expansionof the leaner ones, or three separate absorbers may be used combiningthe rich oil streams to one still. Either way this is carried out willinvolve a complicated system of flash tanks and absorbers to reduce adistillate to final products after such distillate has been recoveredfrom the well stream by re, frigerated retrograde condensation.

The present invention accomplishes the removal of methane without lossof polymerization feed stock, by making use of desorbers operating ondistillate instead of an absorption oil and applying the principle ofpreferential absorption of one component over another when contactedwith an absorbing medium. Desorption alone cannot be very successfullycarried out on these refrigerated distillates because their ethanecontent is high and desorption is most efficient on methane when usingethane vapor as a desorbing medium. The high content of heptane andheavier, which may increase if a two-phase system comes into theproducing reservoir, favors desorption by reason of its higher molecularweight. Heptane and heavier are an impediment to conventionalrectification or oil absorption and an increase in these componentswouldseriously disturb operation of rectifying equipment, whereas theequipment of the present invention would operate without change in caseof such an increase in the percent of heavy ends.

The cycle described herein is not a substitute for conventionalrectification, but a method for the entire elimination of methane at ahigh pressure, around 500 to 600 pounds per square inch, and thesegregation of ethane at a similar pressure so that conventionalrectification will deal with a liquid comprising propane and heavier. Apart of the propane, together with the ethane, is delivered as a liquidfor further separation or for cracking to ethylene.

One of the objects of the present invention is to provide an improvedprocess for the removal of undesirable lighter components from a fluidmixture.

A further object of the present invention is to provide a processparticularly adaptable to rectification of condensate from distillatetype wells whether made by reduction of pressure and cooling or as aresult of high pressure oil absorption.

A still further object of the present invention is to provide a processwhich, when applied to rectification of condensate, will eliminatemethane and at the same time retain ethane and heavier.

Other objects and advantages will be apparent from the followingdetailed description, and from the accompanying drawing.

The accompanying drawing is a diagrammatic elevation of an apparatussuitable for carrying out the present invention.

Referring to the drawing, the numerals I, 2 and 3 refer to a series ofdesorbers in combination with a stripping column 4 and a fractionator 5.Desorbers I, 2 and 3 may consist of plate columns containing six toeight plates and having some empty shell near the top where flashingoccurs. Pipe 6 introduces feed to the system from which it may be fedthrough valve 1 to desorber I, or through pipe l3 and valve M todesorber 2. A pipe 8 is arranged at the upper end of desorber I having aback-pressure regulator 9. Pipe Ill enters the lower part of desorber Ifor admission of vapor from the top of desorber 2. Connected to the baseof desorber i is a pipe H connected with a float controlled valve l2 andjoining pipe l3 which enters the flash section of desorber 2. Vapor fromthe top of desorber 2 may be withdrawn through pipe l8 and valve [8' orcompressed by compressor H) for blowing the base of desorber l. Pipe l6admits vapor to the base of desorber 2 while liquid from desorber 2 iswithdrawn through a float controlled valve I1 and a cooling coil l8 tobe admitted to the flash section of desorber 3. A vapor pipe l9 providedwith a back-pressure regulated valve 28 conducts vapors from the top ofdesorber 3 to compressor 2|. The base of desorber 3 is blown with vaporentering through pipe 22 and discharges liquid through float controlledvalve 23 and pipe 24 to the feed plate of plate-type stripper 4. Vaporgoing overhead from stripper 4 passes through back-pressure regulator 25to compressor 26, where it is compressed to the pressure of desorber 3,thence through cooling coil 21 to separator 28. Liquid condensed fromthe vapor stream by compression and cooling is admitted as reflux tostripper 4, the flow of reflux being regulated by float-controlled valve28. Uncondensed vapor from separator 28 blows the base of desorber 3.Float-controlled valve 3!! at the base of stripper 4 effects withdrawalof product A which is sent to a conventional rectifying column. Vaporfrom lower plates of fractionator is conducted to the base of thestripper by pipe 3| where its admission is regulated by athermostat-controlled valve 32. Vapor from desorber 3 is compressed bycompressor 2| and fed hot to the feed plate of a plate type fractionator5 through pipe 34. High pressure fractionator 5 has a steam heating unit35 incorporated in its base. Vapors taken overhead from column 5 by pipe38 are conducted through cooling tower 37 and sent to accumulator 38.The uncondensed vapors from accumulator 38 ,pass through back-pressureregulator 39 and are conducted by pipe l8 to the base of desorber 2.Reflux pump 48 returns liquid from the bottom of the accumulator tofractionator 5 as a reflux which is regulated by flow control At. Afloatcontrolled valve 12 regulates withdrawal of product B from thebottom of fractionator 5 which is: passed to a conventionalrectifyingcolumn. Float-controlled valve 43 effects with drawal of product C fromaccumulator 38.

For the purpose of illustration, a typical condensate will be taken asan example showing how this condensate may be processed eliminatingmethane at high pressure for recycling and conservingpolymerization-feed stock for further processing. Condensate from ahigh-pressure accumulator is delivered to the system at 1400 pounds persquare inch and 78 F., having the following composition:

Mol per cent Methane 37.08

Ethane 15.15 Propane 7.89 Butane 8.19

Pentane 5.99 Hexane 5.39 Heptane plus 20.31

The sum of the partial vapor pressures of the ethane and heavier in thisexample at 150 F. is approximately 4'75 pounds per square inch. In thisparticular example, the use of desorber l is not necessary to effect thecomplete removal of methane. The condensate is, therefore, bypassedthrough pipe l3 and valve [4 to desorber 2. To remove all of the methanein this desorber it is necessary to use the combined effect of addingsome ethane at its base to displace methane plus the heat of solution ofethane so that the combined vapor pressure at its base in the absence ofmethane is equal to the vapor pressure at its top in the presence ofmethane. This desorber is operated at a pressure slightly in excess ofthe vapor pressure of the ethane and heavier or at a pressure of 500pounds per square inch. Methane isv removed through pipe I0 and valve l0 to compressors for recycling to the producing formation. Desorber 3 isoperated at a pressure of about 250 pounds per square inch and is blownwith the uncondensed vapor taken from separator 28 which operates at thesame pressure. Stripper 4 is operated at a pressure of about 60 poundsper square inch and is refluxed by. the condensate from separator 28.Vapors from the top of desorber 3 are compressed to 500 to 600 poundsper square inch and fed hot to fractionator 5 which operates in thatpressure range. Fractionator 5 is a conventional rectifying column withvapor feed. Its kettle is steam heated and it is refluxed in theconventional manner. It is operated with regulated heat input at itsbase with a bottom product (product B) free from ethane and consistingchiefly of propane with minor amounts of butane and heavier. Its refluxat 500 pounds per square inch is about. 75% ethane and 25% propane.Excess reflux or product C of the same composition as the reflux iswithdrawn as a liquidfrom accumulator 38 for rectification,polymerization, alkylation, or cracking, depending upon thecircumstances. Vapors from accumulator 38- containing to ethane,together with some propane and heavier, are sent to desorber 2 todisplace methane.

Reflux to fractionator 5 is regulated by flow controlled valve 4|, sobalanced withheat input as to give a bottom product ethane free.Backpressure regulator 39 is a control for the entire desorbingoperation. It is set by the operator at a pressure which, at thecondenser temperature, will condense the ethane in the original feed together with a part of the propane also present. If methane is notentirely removed in desorber 2, pressure will tend to rise onaccumulator 38, sending more desorbing vapor to desorber 2. Theoperating pressure of fractionator is higher than the pressure ofdesorber 2 to permit flow of vapor, its pressure being determined bythat of the desorber. Stripper 4 is blown by vapor, mainly propane,taken from the base of fractionator 5 by pipe 3 I the adjustment beingsuch as to give freedom of ethane to the base of the stripper. Product Afrom the base of the stripper is rich in the heavy components and may besent to a lower plate of the conventional rectifying column to whichproduct B is sent.

To avoid excessive load on the high pressure compressor 2|, the pressureon the desorber which completes the removal of methane, desorber 2 inthis example, should not be much higher than the sum of the vaporpressures of components other than methane at the temperature of thebase of that desorber. sure of the high pressure desorber cannot beraised indefinitely to give a leaner gas from its top. If the characterof the condensate feed is such that desorber 2 will not give goodseparation of methane, desorber I is used to remove a part of themethane at a higher pressure. In using desorber l, valves l5 and I0 areclosed and the feed admitted to the flash section of desorber I throughvalve 7. Methane is withdrawn through pipe 8 and back-pressure regulator9 at the top of the desorber. Some of the methane of the original feed,together with the heavier components passes as a liquid through pipes Hand I3 to the flash section of desorber 2. The final removal of methaneis accomplished in desorber 2; the methane, together with some of theheavier components leaving desorber 2 through pipe l0 enters compressorl5, from which it is sent to the base of desorber I.

Now in case of the material given as an example, the bottoms fromstripper 4 will contain over 40 mol per cent of heptane and heavier. Tofeed such material to a rectifying column for the separation of propaneas a top product will give an unreasonably high kettle temperature.Though this is no part of the invention, the bottoms of stripper tshould be fed instead to a column whose tops are a mixture of propaneand butane and to a higher feed plate in this same column should be fedliquid bottoms from column 5. This column should operate with topstotally condensed and these tops be pumped to a column at higherpressure having propane tops and butane'bottoms.

By combining the methods of desorption and rectification, methane isremoved at high pressure while retaining ethane in solution in theoriginal liquid. To remove methane at a really high pressure, two thingsare necessary: (1) that the original liquid contains enough ethane sothat its methane free vapor pressure be relatively high and thesedistillates answer that requirement and (2) that the displacing vaporwhich desorbs the methane should be very high in ethane content so thata minimum amount of it be required, thus minimizing the compressor load.The high pressure rectifying column gives this around 95% ethane vaporfor use as high pressure desorbing vmediiun for methane. It also gives areasonably high propane vapor from its base as the final de- In otherwords, the pres-,

sorbing medium for ethane in the low pressure stripper.

The reason desorption alone is not well adapted to removal of ethane, isthe relatively high solubility of ethane in all heavier hydrocarbonsfrom a vapor phase rich in ethane. While it is quite simple to desorbethane on the lower plates of a low pressure desorber, it is quitediflicult and really impractical to get it out of a desorbing systemthrough the top plates of a high pressure desorber. Of course, this factis exactly what makes ethane so useful as a desorbent vapor for methanein solution.

This difiiculty is overcome in the cycle here described by letting theethane out of the system by way of the high pressure rectifying column.Extreme high pressure on this column is avoided by including propane assolvent for ethane in the reflux and top product. It should also benoted that if it be desired to condense pure ethane to the liquid form,this cycle provides the intermediate product, a mixture containingethane and propane only, in the rectification of which the propane askettle product can be boiled at reasonably low kettle temperature at thepressure necessary to form an ethane reflux.

It is to be understood that the operating pressures given areapproximate, and are selected for the distillate herein taken as aspecificexample. Optimum conditions of pressure, temperature, etc., maybe calculated by methods well known to those skilled in the art, theactual operating pressures being determined by the operator. It has beenstate-d that the pressure of the desorber which completes removal of,methane should be only a little above the vapor pressure of thecomponents heavier than methane in the original feed at the temperatureon the bottom plate of the desorber. The pressure on stripper 4 whichcompletes removal of ethane should be somewhat below the vapor pressureof propane and heavier in the original distillate, which is 65 poundsper square inch at F. for the example taken herein. The pressure ofdesorber 3 (shown at 250 pounds per square inch) is an intermediatepressure, chosen to minimize butane and heavier in its top vapor andhence in the kettle of the high pressure rectifying column. The bestvalues of these pressures is determined during operation as those whichresult in the smallest load on the compressors.

I claim:

1. The process of treating distillate or condensate from high pressurewells comprising passing the distillate through a methane desorbingzone, passing the bottoms from this desorbing Zone through a seconddesorbing zone to remove ethane, propane and heavier vapors, passing thebottoms from this second desorbing zone to a stripping zone and removingtherefrom stripped distillate or condensate as bottoms and a vaporousoverhead, passing said vaporous overhead to the second desorbing zone;passing the ethane, propane and heavier vapors from the second desorbingZone to a fractionating zone, removing from the said fractionating zonean ethane-free bottoms, a vaporous side stream and a vaporous overhead,condensing said fractionator overhead to form a polymerization feedstock and passing the uncondensed portion to the methane desorptionzone, and passing said vaporous side stream to the stripping zone asstripping agent.

2. The process of treating distillate or condensate from high pressurewells comprising passing the distillate through a methane desorbing zoneincluding a plurality of methane desorbing steps of successively lowerpressures, passing the bottoms from the last of the methane desorbingsteps through a second desorbing zone to remove ethane, propane andheavier vapors, passing the bottoms from this second. desorbing zone toa stripping Zone and removing therefrom stripped distillate orcondensate as bottoms and a vaporous overhead, condensing said vaporousoverhead and passing the condensate to the stripping zone as reflux andthe uncondensed portion to the second desorbing zone; passing theethane, propane and heavier vapors from the second desorbing zone to afractionating zone, removing from the said fractionating zone anethane-free bottoms, a vaporous side stream and a vaporous overhead,condensing said fractionator overhead to form a polymerization feed stopand passing the uncondensed portion to the last of the methane desorbingsteps, and passing said vaporous side stream to the stripping Zone asstripping agent.

3. The process of treating distillate or corn densate from high pressurewells comprising passi propane and heavier vapors from the seconddesorbing zone and passing the compressed vapors to a fractionatingzone, removing from the said fractionating zone an ethane-free bottoms,a vaporous side stream and a vaporous overhead, condensing saidfractionator overhead to form a polymerization feed stock and passingthe uncondensed portion to the last of the methane desorbing steps, andpassing said vaporous side stream to the stripping zone as strippingagent.

4. The process of treating distillate or condensate from high pressureWells comprising passing the distillate through a methane desorbing Zoneincluding a plurality of methane desorbing steps of successively lowerpressures, cooling the bottoms from the last of the methane desorbingsteps and passing the said cooled bottoms to a second desorbing zone toremove ethane, propane and heavier vapors, passing the bottoms from thissecond desorbing zone to a stripping zone and removing therefromstripped distillate or condensate as bottoms and a vaporous overhead,condensing said vaporous overhead and passing the condensate to thestripping zone as reflux and the uncondensed portion to the seconddesorbing zone; compressing the ethane, propane and heavier vapors fromthe second desorbing zone and passing the compressed vapors to afractionating zone, removing from the said fractionating zone anethane-free bottoms, a vaporous side stream and a vaporous overhead,condensing said fractionator overhead to form a polymerization feedstock and passing the uncondensed portion to the last of the methanedesorbing steps, and passing said vaporous side stream to the strippingzone as stripping agent.

SAMUEL C. CARNEY.

